Ecology of the mexican alpine blotched garter snake (Thamnophis scalaris)

We report the first information on seasonal abundance, sex ratio, body size, reproduction, and diet for a population of Thamnophis scalaris from the State of Mexico. Data were collected during 1992 and from 1998 to 2000 at Toluca. Thamnophis scalaris showed a bimodal peak of activity in spring-early summer and in autumn. The sex ratio of adults was significantly different front 1:1, but can be misleading because of the unknown natural history of T scalaris. Sexual size dimorphism was only apparent in relative tail length of adults. Four litters were born from July 1 to 17. Mean litter size was 7.0 +/- 1.4 snakes per litter. Male and female neonates were of similar snout-vent length (SVL) and mass at birth. The sex ratio of neonates was 1:1. Of the stomachs examined, 81 % contained earthworms and 19% contained vertebrates. Vertebrates (lizards and viperine snakes) were ingested only by T scalaris > 40.0 cm SVL. This suggests an ontogenetic shift in the diet of T. scalaris. The mean vertebrate prey mass was 21.4 +/- 10.0% of snake mass. No differences were found in prey type between the sexes.